SUMMARY

Plant stem water potential was monitored continuously and non-destructively on both woody and herbaceous species under partially controlled greenhouse environment and/or field conditions. The in situ stem psychrometer was automated under computer control which resulted in 30 minute time resolution on measurements of stem water potential. These data provided greater time resolution of plant water potential than has been previously attained. Concurrent measurements of ambient vapour pressure deficit and solar radiation were also made. Interactions among physiological and environmental variables were interpreted with respect to specific and varietal differences and the extent to which mechanisms of response to water stress could be demonstrated.

Field measurements of stem water potential on tomato (Lycopersicon esculentum L.Var.) implied distinctive varietal differences in the pattern of stomatal response to routine diurnal water stress. Greenhouse experiments on potted tree saplings (e.g. Fraxinus pennsylvanicum, Gleditsia enermis, Betula verucosa) exhibited direct correlations between stem water potential and incoming radiation. Clear differences were exhibited between species with respect to their water status under the same environmental conditions. The relationship between stem water potential and stomatal conductance in red ash indicated that partial stomatal closure was induced when the water stress dropped to about -1.0 MPa. This was followed by some rehydration, an increase in stomatal conductance and a second dehydration phase. The kinetics of water potential changes during progressive dehydration and recovery upon rewatering were observed. The automated in situ stem psychrometer emerged as a reliable and insightful tool for evaluating plant-environment interaction.